F-curves disappeared but keyframes still work
Now that you have turned it into an NLA strip, you have to select the strip in the NLA editor and press TAB to begin editing the action in that strip.  One problem is that if you add keyframes outside the boundaries of the strip they will not affect the object.  There are other complications surrounding the strip extrapolation settings.

Alternately, you should be able to "undo" the stripification by selecting the object and using the dope sheet's Action Editor mode to assign its primary action to the one that was there before the stripification (make sure you are not tabbed into the strip or your change will affect the strip instead of the primary action).  It's the combo box left of the Push Down button.  At that point the object will have a primary action AND an NLA track with strip. You can delete the NLA track by selecting the track in the NLA editor and using the x to delete the track.